1. Technical Field
The present invention relates generally to apparatus and methods for accelerating charged particles in a radio-frequency (RF) cavity. In embodiments of the present invention, a radio-frequency (RF) cavity apparatus may be used to accelerate electrons for various applications, including generation of X-rays and Terahertz radiation.
2. Background Information
Conventional methods of accelerating particles, e.g. electrons for X-ray generation, use a linear particle accelerator to accelerate the particles to energies of several keV or several MeV. In typical electron accelerators, a conventional injector emits an electron beam that is accelerated towards a target (interaction point), where electromagnetic radiation of different spectra is generated by different means. After that, the electron beam is dumped on a collector where X-ray radiation is generated upon impact via bremsstrahlung. Soft X-rays having energies of 0.12 to 12 keV and wavelengths of 10 to 0.1 nm can be generated in this way at either the interaction point or the collector. In more recent times linear accelerators such as the Stanford Linear Accelerator Center (SLAC) typically achieve electron energies around 3 GeV by using radio frequency (RF) fields to progressively accelerate an electron beam as it passes through a accelerating structure containing segmented RF cavities. Such high energy electron beams can be circulated in a storage ring using synchronized electric and magnetic fields and used, for example, to provide a source of synchrotron radiation including X-rays. These extremely bright (i.e. high flux) X-rays can be used to investigate molecular structures, resulting in many bio-medical applications such as protein crystallography.
While light sources such as the one at SLAC and the Diamond light source in the UK can provide researchers with very hard and bright X-rays for experimental studies, such facilities are extremely large, costly to run and not readily available to everyone. The Diamond light source is housed in a toroidal building that is 738 m in circumference and covers an area in excess of 43,300 m2. Although the X-rays from a synchrotron source can be a billion times brighter than those, for example, generated by cathode ray tubes for normal medical imaging, a synchrotron source converts only a tiny fraction of the energy of the electrons into radiation. Furthermore the natural synchrotron light is not monochromatic and its application, for example, to phase-contrast imaging may require the use of sophisticated insertion devices and other techniques. Alternative X-ray sources, and particle accelerators generally, are required that can meet academic and industry demands on a more accessible scale.
One alternative to synchrotron light sources is a linear accelerator (linac)-based coherent light source such as the Linac Coherent Light Source (LCLS) at SLAC. This facility couples a linear particle accelerator with a free electron laser (FEL) to produce intense X-rays. In a free electron laser the electron beam itself is used as the lasing medium. The electron beam from the linac is injected into an undulator or “wiggler”—an array of magnets arranged with alternating poles along the light beam interaction path to slightly wiggle the electron beam transversely and stimulate the emission of coherent electromagnetic radiation in the form of X-rays. FEL radiation is monochromatic and extremely bright—the process of self-amplified spontaneous emission extracting a much greater fraction of the electrons' energy than can synchrotron radiation. In fact FEL X-ray sources can be many orders of magnitude brighter than synchrotron light sources.
Some researchers have demonstrated energy recovery in conjunction with a free electron laser by decelerating the electron beam after it has passed through a wiggler. The ALICE accelerator at Daresbury Laboratory in the UK has coupled an energy recovery linac to the undulator of a free electron laser generating light in the mid-IR range. In such a proposal the spent electron beam is returned back to the entrance of the main linac via an additional beam path at a precise time when the RF phase is exactly opposite to the initial accelerating phase such that the beam is decelerated and energy can be recovered back to the electromagnetic field inside the linac RF cavities. This energy recovery technique requires an accurate adjustment of the electron beam path length that is accomplished by moving the arc of the beam path as a whole.
While accelerators such as the LCLS at SLAC and ALICE at Daresbury Laboratory have demonstrated the potential of FELs as light sources, there are several drawbacks. Such facilities are extremely large—the LCLS based on a linear accelerator at SLAC, for example, is over 3 km long in total and includes a 600 m linac, 230 m electron beam transport tunnel, 170 m undulator and over 300 m of tunnels to transport X-rays to experimental halls. The overall billion dollar-scale cost and huge size of such machines means that they can only be constructed at a national level. There remains a need for smaller research bodies to have access to their own accelerators and smaller scale sources of THz radiation or X-rays.
Researchers at MIT have recently proposed an alternative X-ray source that is potentially smaller than the LCLS or other sources based on the principle of a free electron laser. This alternative technique uses inverse—Compton scattering to generate X-rays when an electron beam is collided with photons e.g. from a laser beam. U.S. Pat. No. 7,391,850 describes such a laboratory scale X-ray source.
WO 2012/061051 describes an X-ray generation apparatus utilizing energy recuperation to improve X-ray generation efficiency. The apparatus generates X-rays by accelerating a beam of electrons using a first RF cavity arrangement and then interacting the electrons with photons to generate X-rays via inverse—Compton scattering. After the interaction with the photons, the electrons are decelerated in a second RF cavity arrangement. The first and second RF cavity arrangements are connected by RF energy transmission means arranged to recover RF energy from the decelerating electrons as they pass through the second cavity arrangement and then to transfer the recovered RF energy to the first cavity arrangement. The apparatus thereby provides an improvement over existing X-ray generation methods as the recuperation of the RF energy improves the efficiency of the X-ray generation.
There remains a need for compact sources that can efficiently generate high energy and high flux X-rays or other radiation for use in a wide range of experiments, in particular to extend the range of experiments that can be conducted using such compact sources. In addition, there remains a need more generally for compact particle accelerators that benefit from using energy recovery and can achieve high operating currents.